MINISTRY OF AGRICULTURE, FISHERIES AND FOOD CSG 15 Research and Development Final Project Report (Not to be used for LINK projects)
Two hard copies of this form should be returned to: Research Policy and International Division, Final Reports Unit MAFF, Area 6/01 1A Page Street, London SW1P 4PQ An electronic version should be e-mailed to [email protected]
Project title Use of a humane method for fox control
MAFF project code VC0322
Contractor organisation Central Science Laboratory and location Sand Hutton York YO41 1LZ
Total MAFF project costs £ 16,880
Project start date 01/01/01 Project end date 30/04/01
Executive summary (maximum 2 sides A4)
1. Neck snaring is the most commonly used method of fox control in the UK. However , the humaneness of
neck snares varies tremendously with the skill of the operator. With growing public antagonism towards the
use of neck snares, humane and efficient alternatives are badly needed.
2. Fumigation of fox earths with carbon monoxide (CO) has been identified as a humane alternative, and is
already being successfully developed for rabbit control. However it is unknown whether the concentrations
of CO needed to obtain a humane kill are achievable within a fox earth with one entrance.
3. In order to ensure humane death the target concentration of CO was 1 % for a minimum of 60 minutes.
4. Two artificial earth’s were built with gas sampling lines inserted into the tunnels at several locations. One
earth had a diameter of 0.3 m, total length of 9.5 m and volume of 0.67 m-3. The second earth had a diameter
of 0.23 m, total length of 10 m and volume of 0.42 m-3
CSG 15 (Rev. 12/99) 1 Project Use of a humane method for fox control MAFF VC0322 title project code 5. The earth’s were built using light sandy soil and were raised above the level of the surrounding ground, thus
mimicking a worst case situation for fumigation.
6. Sufficient cartridges to produce a theoretical concentration of 4 % CO were used. Six cartridges were
ignited in the larger earth and 4 in the smaller earth. The trial was replicated six times for each of the
earth’s.
7. The concentration of CO achievable was weather dependent. During windy conditions (wind speed >5
km/h) the target concentration was achieved in half of the sampling points, whereas during calm conditions
the target was reached in 11 out of the 12 sampling points.
8. Under calm conditions CSL CO cartridges will produce a concentration of CO potentially sufficient to
humanely kill foxes.
CSG 15 (1/00) 2 Project Use of a humane method for fox control MAFF VC0322 title project code
Scientific report (maximum 20 sides A4)
1. Introduction
Neck snaring is by far the most common method of fox control in the UK (79 % of respondents, in a BASC
survey conducted in 1995, listed snaring as the method of fox control they used as opposed to 11 % who listed
cage trapping). However, the humaneness of neck snares, more than any other method of fox control, varies
tremendously with the skill of the operator. Badly sited snares injure non-target species, such as deer and
badgers, and can cause immense suffering to foxes caught in them. It is not uncommon to find dead foxes in
snares with thick suffusions of bloody lymph fluid beneath the skin of the head and neck indicating that the
animal has suffered a slow and very painful death. There is growing public antagonism towards the use of neck
snares such that some political parties have pledged to ban the use of neck snares completely. Yet the banning
of such snares would have a major impact upon fox control. Humane and efficient alternatives are badly
needed. MAFF has a remit to promote the use of humane wildlife management techniques.
Although the use of poison bait is unlikely to be an acceptable method of fox control, fumigation of fox dens
was shown by MAFF trials in the 1960’s to be an effective control method in some circumstances. In these
trials cyanide was generated in the den by the use of Cymag powder that reacted with moist earth. Today
Cymag is approved for rabbit control but no request has been made by the company concerned, for clearance of
this compound for use against foxes. Research conducted by CSL on a range of fumigants, concluded that
although cyanide is a humane poison, it can pose severe safety and environmental problems. Carbon monoxide
was found to be as humane as cyanide but far less hazardous to the user. In addition the CO cartridge
developed by CSL can be safely used in wet conditions unlike Cymag powder. This CO cartridge was
developed by CSL for rabbit control but can be adapted to make it suitable for fox control. In North America
similar cartridges are used to kill coyotes.
CSG 15 (1/00) 3 Project Use of a humane method for fox control MAFF VC0322 title project code CO has been used for animal euthanasia for many years, and as such, much is known about the concentrations
and duration of exposure needed to cause death. In a review of animal euthanasia, Green (1979) reports that
CO concentrations of between 0.5 and 14 % have been used and that the animal is unconscious before showing
signs of stress. Dogs exposed to 0.25 % CO collapsed within 10-15 minutes with no signs of distress (Burrell,
Seibert & Robertson, 1914), though generally higher concentrations (1.9 -8 % CO) have been used for
euthanasia of dogs. After exposure to such concentrations dogs show sudden prostration followed by
myoclonic contractions with vomiting, urination and defaecation (De Vries et al. 1977). The EEG from the
dogs indicates that they are in a comatose state from the first behavioural signs of intoxication i.e. falling down,
and therefore the dogs do not suffer any distress before unconsciousness. Signs of distress can be observed if
the concentration of CO is raised too rapidly. For instance, pigs exposed to rapidly rising concentrations of CO
experienced vigorous convulsions and vocalised extensively for 1 minute before lapsing into a coma (Lambooy
& Spanjaard, 1980). CO concentration was not critical. A previous report into the toxicology of CO to
mammals recommended that any use of CO for fumigation of mammals should ensure exposure to
concentrations greater than 1% and to gradually increasing concentrations (Ross, 1993). This should prevent
the onset of convulsions before insensibility.
The technical problem to be addressed in this research concerns the practicality of generating in a fox den the
concentration of CO that will result in the humane death of a fox. This research will specifically measure
whether the use of several cartridges previously developed by CSL for rabbit fumigation can generate the
concentrations of CO required in an artificial fox earth.
2. Materials and methods
CSG 15 (1/00) 4 Project Use of a humane method for fox control MAFF VC0322 title project code 2.1 CO cartridges
The cartridges were made by packing a mixture of prilled sodium nitrate (Chilean, Ellis Everard, London;
particle size 0.85-1.70 mm), activated charcoal (Merck Chemicals, Poole) and powdered zinc oxide (laboratory
reagent grade; Merck Chemicals, Poole) inside a cardboard tube (71 x 40.5 mm diameter) equipped with two
cardboard closures, one of which had provision for 4 gas outlets and one hole for insertion of a fuse. The fuse
was made from two lengths of plastic igniter cord (Nobel Explosives, ICI) twisted into a tuning fork
configuration and inserted into the cartridge as it was filled. The proportions of ingredients used was 54.2 %
sodium nitrate, 29.2 % charcoal and 16.7 % zinc oxide, with each cartridge weighing approximately 700 g.
Multiple cartridges were joined together by using longer pieces of igniter cord twisted around the length
protruding from the top of the cartridge.
2.2 Artificial Earths
Two artificial earths (Figure 1) were built based on a published design recommended for housing foxes (Holb
1996) and after discussion with fox experts (D. O’Connel & J. Woods, personnel communication). An
important modification from the published design was the diameter of the tunnels. One earth (earth B) had a
diameter of 30 cm representing tunnels excavated by badgers and the other (earth F) a diameter of 23 cm,
approximately the size usually excavated by foxes. Earth B consisted of 9.5 m of tunnel with one entrance and
3 blind ends, earth F had 10 m of tunnel again with one entrance but only 2 blind ends. The tunnels were
constructed from netlock wire wrapped in hessian, which were then placed on the ground and then covered with
soil to a depth of 0.5 m and width of 0.5 m at either side. The soil was a mixture of 50 % top soil and 50 %
sand, thereby mimicking the worst case situation for fumigation i.e. a light sandy soil.
2.3 Measurement of CO concentration
Nylon tubing (3 mm diameter) was inserted into the tunnels at several locations in order to be able to extract
gas for analysis. CO concentration was measured using both gas analyser types with Gastec pumps and gas
chromatography (GC) equipment housed in a mobile laboratory. A Hewlett Packard 5880 gas chromatograph,
CSG 15 (1/00) 5 Project Use of a humane method for fox control MAFF VC0322 title project code with a 12’ x 18” column packed with sperocarb 80/100 mesh, was fitted with a thermal conductivity detector.
The oven temperature was 100°C and the carrier was 20 ml min-1 of helium. The GC was calibrated using a
standard mixture of 5 % CO in nitrogen (Bedfont Instruments). The gas-sampling lines for the chromatograph
were connected to a 16-port stream selection valve and samples were drawn by diaphragm pump at a rate
determined by a mass flow controller. The samples were injected onto the column using a 0.25-mL automatic
sampling loop. To prevent interference from CO2 and water vapour, small guard tubes packed with carbosorb
(AS) were fitted in the gas sampling lines. The accuracy of the gas analyser tubes was checked using
simultaneous measurements by analyser tube and GC.
2.4 Protocol
Initially, pilot trials were completed with 2 and 4 cartridges in earth F and earth B, respectively. After
preliminary examination of the results the number of cartridges used in each earth was increased to 4 and 6,
respectively, for the trials reported here.
Six replicate trials were conducted in each of the earths. The CO cartridges were placed approximately 15 cm
inside the entrance of the earth within a foil tube (20 cm diameter) with the perforated end pointing into the
tunnel. The foil tube was used to prevent burning of the hessian and collapse of the tunnel. The fuse was
ignited and the entrance to the earth immediately sealed with sand bags. CO concentrations were measured for
a minimum of 2 hours, approximately every 50 minutes from each point. Wind speed and direction were
recorded manually every 15 minutes with a hand held anemometer 1.5 m above ground level at the entrance to
the earth under trial.
3. Results
It was clear from the results that wind speed affected the distribution of CO within the earths. When conditions
were windy the CO diffused out of the earths relatively quickly and was not dispersed effectively within the
earth. The effects of wind were examined by separating the results of the trials conducted in calm conditions or
light winds (<5 km h-1) from trials conducted in windy conditions (> 5 km h-1). Of the six replicate trials
CSG 15 (1/00) 6 Project Use of a humane method for fox control MAFF VC0322 title project code carried out in earth B, 4 were under windy conditions and 2 were under calm conditions. In earth F, 3 replicates
were completed under windy conditions and 3 under calm conditions.
The sample points used differed in either distance from the entrance to the earth or complexity of tunnel that the
gas had to pass down to reach that point. The carbon monoxide concentrations reached at each sample point are
therefore, all presented separately. The results are shown in figures 2 to 25.
High CO concentrations (9.4-13.4 %) were recorded at the entrances to the earths in the first 15 minutes after
treatment but decreased rapidly (Figure 2, 3, 14 and 15). After 6 hours the CO concentration was still greater
than 0.5 % in the entrance to earth B during calm conditions (Figure 2) but during windy conditions dropped
below this concentration after 3 hours (Figure 3). In earth F weather conditions had no effect on the speed of
dissipation of the gas, CO concentrations dropping below 0.5 % at the entrance after approximately 3 hours
(Figures 14 and 15).
CO concentrations were lower within the tunnel system when compared to the entrances, with the lowest
concentrations being found at points furthest from the entrances i.e. sampling points 1 and 8 (Figures 10, 11, 24
and 25). CO concentrations peaked at all sampling points within 90 minutes of the start of all trials. Although
the CO concentrations found within the tunnels were low, they were found to be above 1 % for longer than 60
minutes at all sampling points apart from point 8, within both earths when wind conditions were calm (Figure
26 and 27). During windy conditions the target concentration was reached at four out of the 6 sampling points
in earth B, but only two for earth F. The time for which CO concentration was higher than 1 % for more than 60
minutes was significantly less often during windy conditions than calm conditions for both earths (Mann-
Whitney U test, Earth F; Z=3.94, p=0.01: Earth B; Z= , p=0.026).
4. Discussion
CSG 15 (1/00) 7 Project Use of a humane method for fox control MAFF VC0322 title project code Even though higher concentrations of CO throughout the earths were found with the increased number of
cartridges, the target concentration of > 1 % for at least 1 hour, was not met during all trials at all sampling
points. Each cartridge has been found to produce approximately 5 l of CO (Page, 1993). Distributed evenly
throughout the earths, with no loss out of the tunnel, this would amount to a maximum achievable concentration
of 4 %. Higher concentrations would be expected to occur in the vicinity of the cartridges before the gas has
been fully distributed through the earth. On calm days CO concentration reached 1 % for longer than 60
minutes at all sampling points except point 8 within earth F. Point 8 was the furthest from the entrance and we
therefore expected to measure the lowest peak in CO concentration at this sampling point. The earths used for
this trial were above ground level and also constructed from sandy soil both factors which would allow the
wind conditions to have a dramatic effect on the CO concentrations. Earths in the countryside are generally
underground and would probably be less susceptible to the effects of wind.
Exposure to concentrations of CO between 0.5 % and 1 % for approximately 80 minutes causes death of
animals, however during this time the animal could possibly experience side effects such as convulsions,
headache and nausea. Concentrations of CO greater than 1 % lead to loss of consciousness occurring before
these side effects become evident in pigs, and is therefore the concentration to aim for. During calm conditions
the CO cartridges would be expected to result in the humane killing of any fox within the earth. Although it is
unknown how a fox would behave after initial exposure to CO, movement within the tunnels would probably
cause the fox to be exposed to a greater concentration of CO rather than a lower one.
When carrying out fumigation of a fox earth the approximate size of the earth and the wind conditions would
need to be assessed to determine how many CO cartridges would need to be used. Using more cartridges when
conditions were windier may result in high enough concentrations being reached under such conditions.
However, further trials would be needed to verify this. In addition using extra cartridges would increase the
rate of CO production as well as raising the final concentration reached. Raising concentrations of CO too
quickly has been shown to cause distress in pigs, therefore the effect of adding more cartridges on the welfare
CSG 15 (1/00) 8 Project Use of a humane method for fox control MAFF VC0322 title project code of the foxes in the earth would need to be assessed. As the foxes can move down the tunnels and away from the
primary source of CO the rate of change experienced by the foxes may not be increased substantially.
Further trials to determine the success of this technique to humanely kill foxes in initially an artificial earth and
secondly a real earth would need to be undertaken before this method could be advocated as a new method of
fox control. However, until a reliable method becomes available of determining whether or not a badger is
present down a fox earth such trials will not be undertaken. Hopefully the acoustic sensor currently under
investigation in a new DEFRA funded project will provide a reliable method of badger and fox detection.
5. Conclusion
The trials reported here have shown that under calm conditions CSL CO cartridges will produce a concentration
of CO potentially sufficient to humanely kill foxes.
6. References
Burrell, G.A., Siebert, F.M. & Robertson, I.W., 1914, Relative effects of carbon monoxide on small mammals.
US Dept. of the Interior Bureau of Mines Tech. Paper No. 62, 23pp.
de Vries, H.W., Zimmerman, A.N.E., van Leeuwen, W.S., Maas, A.H.J, Douze, J.M.C. & de Leeuw, R.J.M.
1977. An experimental study of acute carbon monoxide intoxication in dogs. Acta Pharmacol. Toxicol. 41
(suppl. 2.) 374-392.
Green, C.J. 1979. Animal anaesthesia. Laboratory Handbook 8. London. Laboratory Animals Ltd.
Kolb, H., 1996. Country Foxes. Whittet Books.
Lambooy, E. & Spanjaard, W. 1980. Euthanasia of young pigs with carbon monoxide. Vet. record 107, 59-61. CSG 15 (1/00) 9 Project Use of a humane method for fox control MAFF VC0322 title project code
Page, R.J.C., 1993. Production of a Carbon Monoxide fumigant cartridge for use in rabbit control. Project
report to MAFF, Project No. PV0302.
Ross, J. 1993. A review of the toxicology of carbon monoxide to mammals. Project Report to MAFF, Project
No. PV1703.
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